1. Field of the Invention
This disclosure relates generally to a system for offloading oil from an offshore Floating Production Storage and Offloading Unit (FPSO) or Floating Storage and Offloading Unit (FSO). More particularly, it relates to an apparatus, systems, and methods for mooring an offloading vessel to an offshore FPSO or FSO.
2. Background of the Technology
Floating Production Storage and Offloading units (FPSOs) and Floating Storage and Offloading Units (FSOs) are commonly used in offshore oil and gas operations to temporarily store and then offload produced oil. An FPSO vessel is designed to receive crude oil produced from a nearby platform or subsea template, process the crude oil (e.g., separation of water from the crude oil), and store the processed oil until it can be offloaded to a tanker or transported through a pipeline. An FSO vessel is a simplified FPSO without the ability to process crude oil. An FSO typically receives and stores oil that has already been processed, and then offloads the stored oil to a tanker or through a pipeline. FPSOs and FSOs are particularly suited in frontier offshore regions where there is no pipeline infrastructure in place for transporting produced oil to the shore. In particular, the FPSO/FSO is employed to store the produced oil until it can be offloaded for transport to another location.
Typically, FPSOs and FSOs are ship-shaped floating vessels or barges that are moored to the sea floor. A plurality of production lines and a plurality of offloading lines are connected to a turret mounted to the bow of the FPSO/FSO. The production lines supply oil to the FPSO/FSO and the offloading lines offload oil stored in the FPSO/FSO to a tanker. A mooring system connects the turret to the sea floor, thereby mooring the FPSO/FSO. In some cases, a hawser also connects to the turret to moor another offshore vessel to the FPSO/FSO. Thus, the turret provides a strongpoint and mounting unit for mooring the FPSO/FSO, while also providing a structural support for the supply lines and the offloading lines. As such, a turret is an important, but complex and expensive component of a conventional FPSO/FSO. For some deepwater applications, the design parameters of the turret may need to be extremely large, bordering on concept feasibility.
There are a few different ways to offloading a moored FPSO/FSO. One approach is to utilize an offloading buoy that is moored to the sea floor and connected to one or more of the offloading lines of the FPSO/FSO. An offloading tanker moors itself to the buoy and weathervanes about the buoy. A conduit extends from the tanker to the buoy and offloads oil supplied to the buoy via the FPSO/FSO offloading line. Since this method employs two floating structures moored to the sea floor (i.e., the FPSO/FSO and the buoy), it can be relatively complex and expensive to implement. Further, since offloading lines extend between the buoy and the FPSO/FSO and the tanker weathervanes about the buoy, there is a possibility of the tanker (or conduits extending between the tanker and the buoy) interfering with the FPSO/FSO (or the offloading lines extending from the FPSO/FSO and the buoy).
Another conventional approach is to directly connect one or more flexible offloading lines of the moored FPSO/FSO to a tanker. The tanker may be moored to the FPSO/FSO with a hawser connected to the turret of the FPSO/FSO and allowed to weathervane about the FPSO/FSO, or the tanker may maintain its position with a dynamic positioning system (DPS). This approach eliminates the need for a second structure moored to the sea floor (i.e., offloading buoy), but has its own set of unique challenges. Specifically, regarding the first method, the turret must be designed to allow the offloading lines to pivot or rotate as the tanker weathervanes. This added functionality may increase the complexity, and associated cost, of the turret. For the second method, the tanker must be equipped with a DPS, which typically includes a global positioning system mated with a plurality of thrusters that work together to maintain the tanker in a specific position for offloading. However, some of the larger tankers such as Very Large Crude Carrier (VLCC) tankers are typically not outfitted with a DPS, and therefore, may not be suitable for offloading in this manner.
Accordingly, there remains a need in the art for improved systems and methods for offloading oil from an FPSO or FSO. Such systems and methods would be particularly well-received if they eliminated the need for DPS, and thus, were suited for use with VLCC tankers, and/or did not require the use of a turret.